In the present study, we evaluated the number of attempts required to master specific subskills for working with whole and rational numbers among students at risk for math difficulties. Participants included a subset of students in grades four through eight receiving supplemental math support. Mastery--defined as 85% correct on short tests--was assessed following instruction for each subskill. Using survival analysis, we evaluated the number of attempts required to reach a 0.50 and a 0.90 probability of mastery on each subskill. The number of required attempts varied across subskills, with many subskills requiring more than one attempt to demonstrate mastery. Further, some of the most difficult content was aligned with curricular standards below students' grade level. Thus, among students identified for supplemental support, it may be worthwhile to remediate select subskills that fall outside of the grade-level curriculum before providing additional instruction on grade-level content. Implications for math subskill assessment and remediation are discussed along with limitations and directions for future research.